The present invention relates to an organic friction material for use in brake shoe and clutch plate of motor vehicles and brake shoe in railway rolling stocks of which the binding material is a resin.
Hitherto, the leading fraction of the organic friction material has been the so-called asbestos-based organic friction material consisting of chrysotile asbestos (hereinafter referred to simply as asbestos) as the reinforcing base material and phenolic resin as the binding material added with various friction-controlling agents.
However, although asbestos has various excellent specific properties, the harmfulness of asbestos dust to human bodies has been recently pointed out and the use of asbestos has come to be restricted from the viewpoint of environmental problems.
On the other hand, as the transport facilities such as motor vehicles and electric cars become to travel at a high speed recently, the demanded properties for braking materials have been more raised than the conventional ones. Particularly, it is demanded that braking effectiveness is excellent and stabilized even under the severe conditions from the viewpoint of safety, and that the braking materials should be longer lasting from the viewpoint of maintenance. Various materials have been proposed for responding these demands.
For instance, a number of the so-called semi-metallic friction materials comprising a metal as base material and phenolic resin as binding material have been developed (U.S. Pat. No. 3,434,998 and U.S. Pat. No. 3,835,118). These semi-metallic friction materials comprise metal or metal oxide as the base material and phenolic resin as the binding material, and generally graphite is used abundantly as the friction adjusting material. Owing to its lubricating specificity, graphite reduces the friction coefficient a little, however, it is said that graphite has a function of suppressing the noise generated at the time of braking and a function of improving the wear resistance.
On the other hand, British Pat. No. 1,387,232 discloses the solution of the problem of low friction coefficient of graphite by substituting carbon in non-graphite form for graphite.
As the result of studying the function of carbon or graphite in such semi-metallic friction materials, the present inventors have confirmed the effect of carbon fibers added to the friction material, and have obtained U.S. Pat. No. 4,278,584. The present inventors have continued the study and have confirmed the effect of the coexistence of carbon fibers and activated carbon in the friction material, and have attained the present invention.
The object of the present invention is to provide a friction material without using asbestos. Another object of the present invention is to provide a friction material having a stable friction coefficient and excellent in anti-fading property (fading property means the phenomenon of reduction of the friction coefficient due to the heat generated at the time of braking) and excellent wear resistance.